pcp-collectl.py: fix problem with -a argument parsing
pcp-collectl.py: fix problem with -a argument parsing

The set of command line options that were subject to "override"
handling was wrong, and there was an attempt to handle -a in the
mainline, rather than deferring most of the work to the option
parsing in libpcp.

With this change, qa/850 passes.

edwardt/pcp,tjanez/pcp,wuliming/pcp,aeg-aeg/pcpfans,edwardt/pcp,aeg-aeg/pcpfans,aeg-aeg/pcpfans,tjanez/pcp,wuliming/pcp,wuliming/pcp,adfernandes/pcp,wuliming/pcp,adfernandes/pcp,edwardt/pcp,edwardt/pcp,prasincs/pcp,tjanez/pcp,aeg-aeg/pcpfans,edwardt/pcp,edwardt/pcp,edwardt/pcp,wuliming/pcp,aeg-aeg/pcpfans,aeg-aeg/pcpfans,wuliming/pcp,adfernandes/pcp,adfernandes/pcp,tjanez/pcp,prasincs/pcp,edwardt/pcp,adfernandes/pcp,adfernandes/pcp,wuliming/pcp,tjanez/pcp,prasincs/pcp,adfernandes/pcp,prasincs/pcp,tjanez/pcp,prasincs/pcp,aeg-aeg/pcpfans,wuliming/pcp,tjanez/pcp,prasincs/pcp,aeg-aeg/pcpfans,adfernandes/pcp,tjanez/pcp,prasincs/pcp,prasincs/pcp